[Ada] Improve error messages to include full package name

[Ada] Improve error messages to include full package name

This patch improves error messages in the compiler so that missing
'with' error messages show the complete package name instead of a
limited number of selectors.

gcc/ada/

	* err_vars.ads: Add new error message names and nodes.
	* erroutc.adb (Set_Msg_Insertion_Name,
	Set_Msg_Insertion_Name_Literal): Likewise.
	* errout.adb (Set_Msg_Insertion_Node): Likewise.
	* errout.ads: Likewise.
	* exp_disp.adb (Check_Premature_Freezing): Modify setting of
	Error_Msg_Node_2 to occur directly before Error_Msg call where
	applicable.
	* sem_ch8.adb (Error_Missing_With_Of_Known_Unit): Added to
	handle the printing of full package names of known units.
	(Undefined, Find_Expanded_Name): Replace error printing with
	call to Error_Missing_With_Of_Known_Unit.